It is often desirable to ascertain with some degree of precision the tension in a belt running over two or more wheels and especially the tension in a V-belt spanning a pair of V-belt pulleys in the engine compartment of an automotive vehicle. Such belts are used to drive the generator or alternator of the vehicle, power steering pump, or air-conditioning compressor and a variety of other accessories. Insufficient belt tension may result in inefficient drive of the accessories while excessive belt tension may result in early wear and breakage, damage to shaft pulleys of the driven member and undue stress on the system.
Although belt tension is frequently ascertained by guesswork, there is a well recognized need for more accurate measuring both for the initial tension setting and during diagnostic studies of engine operation or upon belt replacement.
It has already been proposed to provide a device capable of measuring belt tension which comprises a pair of support members carried by a housing and which are placed against one side (generally the upper side) of a belt, while a third pressure or tension member is brought to bear on the opposite side of the belt usually the underside by the force of a spring in the housing which is braced against the latter.
The linerally effective spring thus urges the pressure member against the belt in a manner which tends to flex the latter between the support members, the degree of such flexure being an indication of the tension on the belt which is displayed by an indicating member utilizing a scale and pointer system, for example.
Conventional systems of this type are rather large so that the locations at which the device can be used are limited and universal application in the vehicle field is excluded.
Furthermore, these devices are relatively heavy and have centers of gravity located such that the device may be difficult to hold during measuring operations and to grip for manual removal from the belt and during setting which is commonly required between successive measurements.
Both the size and the weight disadvantages are associated largely with a need for massive housing to be able to support the high spring force which is required at the pressure member.
In devices of this kind, moreover, the measurement is effected by a compensating method which generally involves manual alteration of the spring force until two reference marks are aligned so that a rapid read-out of belt tension is usually not possible nor is a continual read-out given.
It has also been found that such devices have a fixed transmission ratio between the pressure member and the pointer such that the range of measurement between an excessively loose belt tension and excessively high belt tension is limited. This, of course, makes the device relatively imprecise.
In use, earlier belt tension gauges have been found to leave marks on the belt and so deform the belt that kinks may result. This is detrimental to the belt structure and reduces the life of the belt as well as its uniform or smooth operation. The belt marking appears to be a result of the fact that the support members do not allow compensation for variation in the belt length between the support members during a measurement.
In addition, since the spring characteristic varies as a function of the displacement of the pressure member, the earlier belt-tension gauges have been found to be sensitive to belt thickness. Obviously this sensitivity increases as variations in thickness become proportionally high by comparison to the total spring length. As a result smaller units are less accurate than larger units in the conventional devices.
Almost invariably conventional units are asymmetrical in construction so that they cannot be manipulated as readily by both left-handed and right-handed users. Depending upon the scale and pointer construction and orientation, conventional systems of the type described may be inconvenient to read, generally displaying the result in only one or two viewing planes.
Finally, for the reasons already described, manipulation of the devices is fatiguing so that in practice they have found application only during vehicle manufacture, i.e. in the assembly plant or factory, and have not received widespread acceptance in the service industry.